1. Field of the Invention
This invention relates in general to vehicle body panels, and in particular to composite body panels made from laminations of differing materials, specifically, different aluminum alloys.
2. Background Art
It is the conventional practice in the transportation industry to form body panels, such as deck lids and fenders, by stamping sheet metal. Typically, the transportation industry has utilized steel sheet stock for the production of body panels. In particular, the transportation industry has specified high-strength, low-alloy, cold rolled P & O premium steel for body panels. These panels are resistant to denting and provide a quality surface for the application of paint. However, these panels are comparatively heavy, and as such, have an adverse effect on the weight of the vehicle, and, in turn, the fuel economy thereof However, as CAFE (Corporate Average Fuel Economy) requirements become more and more difficult to meet, there has been a need to address various strategies for reducing the overall weight of vehicles. This has resulted in efforts to light-weight the body-in-white, the drive train and suspension components, and the vehicle body panels. Within these three broad categories, the transportation industry has turned from steel to light metals and plastics. Thus, the use of aluminum in the transportation industry has steadily increased. For example, in 1991, the average amount of aluminum in passenger cars was 191 pounds. By 1996, this value grew to 252 pounds, which represents an increase of 32%. A significant amount of light weight aluminum products, particularly sheet products have been used as inner and outer body panels.
As described above, aluminum alloy sheet products have been substituted for steel body panels. Some aluminum alloys have long been identified by the Aluminum Association as 1000, 3000, 4000, 5000, and 6000 series alloys. These alloy series have also been known as, for example, 1XXX, 3XXX, etc. The 1000 or 1XXX series aluminum alloys are characterized by excellent corrosion resistance, high thermal and electrical conductivity, low mechanical properties and excellent workability. Iron and silicon are the major impurities. The 5000 series aluminum alloys possess good welding characteristics and good resistance to corrosion. Magnesium is one of the most effective and widely used alloying elements of the series. The 6000 series aluminum alloys contain silicon and magnesium and are heat-treatable and versatile. The references in this specification to several alloy series are to be understood as those alloy series as defined by the Aluminum Association in its "Aluminum Standards and Data" handbook. To date, in the United States, automakers have approved only the 6xxx T4 and T6 alloys for use in the production of body panels. These heat treated alloys are quite expensive, but they are capable of stretching and forming without stretch and strain deformations, and are likewise suitable for Class A finishes. Other sheet alloy products offer potential, but do present certain drawbacks. For example, it has been proposed to use of the 5000 series aluminum alloys for body panels because it demonstrates better formability with a higher corrosion resistance and lower manufacturing costs than current heat treatable alloys. Unfortunately, the use of these alloys has been hampered by its tendency to produce stretcher strain lines, also known as Luder lines, when stamped into panels. While some 5xxx alloys have been approved for automobile production in Japan, it remains quite difficult to obtain the proper surface finish on this material. As such, these alloys have only been used for certain specialized applications, where finish is not critical.
It has been suggested that the Ludering phenomenon of 5000 series alloys in outer body panels can be combated by several strategies, for example, a continuous annealing line (CAL), obtaining a larger grain size, or applying a pre-strain into the panel before the final stamping form is given to the panel. Unfortunately, all of these strategies are very costly.
While reference is made to automobiles and the automobile industry, it is to be understood that the present invention is not limited to body panels in automobiles, but rather has applications with other areas of stamping and metalworking.